Prep Work: Chop onions and greens; store together. Chop potatoes and prepare beans; store separately. Whisk together yogurt, lemon juice and zest, mustard, salt and tarragon for dressing.
Place potatoes in bottom of steamer basket and place string beans on top. Steam for approx. 10 min., or until beans are soft but not limp.
Remove beans to salad bowl and keep steaming potatoes until easily pierced with a fork, approx. 5 more min. Set potatoes aside to cool, and then add to salad bowl also.
Add tuna, onion, parsley and salad greens to salad bowl.
In smaller bowl whisk yogurt, mayonnaise, lemon juice and zest, mustard, salt and tarragon; then toss gently with salad. Salt and pepper to taste. (Oil and vinegar can also bring out the flavors; add according to taste.)
(adapted from Weight Watcher's 7/11 "Salad Nicoise")